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A088276
Smallest k such that 10^k + r is a prime ending (least significant digits) in r where r is the n-th odd number not divisible by 5. 0 if no such number exists.
0
1, 1, 1, 1, 0, 2, 0, 3, 3, 0, 2, 0, 2, 3, 2, 2, 0, 5, 0, 2, 2, 0, 2, 0, 3, 2, 2, 3, 0, 2, 0, 2, 2, 0, 3, 0, 2, 2, 2, 2, 0, 3, 0, 3, 4, 0, 3, 0, 6, 3, 8, 3, 0, 4, 0, 4, 4, 0, 10, 0, 3, 3, 23, 4, 0, 3, 0, 4, 3, 0, 4, 0, 3, 5, 3, 5, 0, 3, 0, 6, 3, 0, 5, 0, 4, 3, 3, 633, 0, 3, 0, 3, 3, 0, 3, 0, 9, 4, 4, 3, 0, 4, 0
OFFSET
0,6
COMMENTS
Conjecture: (1) The only zero entries are for values of r == 2 (mod 3). (2) There are infinitely many values of k such that 10^k + r is a prime if r is not == 2 (mod 3).
EXAMPLE
The term corresponding to 21 is 3 as 21, 121 both are composite and 1021 is a prime. 1021 = 10^3 + 21 hence k = 3.
CROSSREFS
Sequence in context: A372818 A258817 A004557 * A099838 A369280 A127449
KEYWORD
base,nonn
AUTHOR
Amarnath Murthy, Sep 28 2003
EXTENSIONS
More terms from Ray Chandler, Oct 15 2003
STATUS
approved